Mayenite is a cement mineral naturally produced in Mayen in Germany, and its crystal structure belongs to a cubic system. The typical composition of the mayenite type compound is 12CaO.7Al2O3 (hereinafter referred to as C12A7) or 12SrO.7Al2O3 (hereinafter referred to as S12A7), or a mixed crystal composition thereof. However, part of Ca or Sr may be replaced by an alkali metal or alkaline earth metal such as K, Na, Li, Mg or Ba or part of Al may be replaced by a metal element having an ion radius of from about 0.5 to 0.8 Å such as Si or Ge.
Ca or Al is a common component for ceramic materials and has been used mainly as one component for structural materials. Usually, an oxide of a metal in or before the third period, including such a compound, is electrically dielectric and shows no electroconductivity.
Heretofore, oxide ceramics showing electroconductivity were ones containing a large amount of an oxide of a transition metal or a typical metal in or after the forth period of Periodic Table of Elements, whereby the environmental load was high.
A crystal of the mayenite type compound has, in its crystal lattice, fine voids (cages) having a diameter of 0.6 nm at a rate of 12 cages per unit lattice, and a C12A7 crystal as its typical composition contains two O2− ions per unit lattice in the cages. Namely, the C12A7 crystal is represented by [Ca24Al28O64]4+.2O2−, and such O2− ions are called “free oxygen” as they are weakly bound (Non-Patent Document 1).
Further, a crystal represented substantially by [Ca24Al28O64]4+.4F− or [Ca24Al28O64]4+.4Cl−, having such free oxygen replaced by fluorine or chlorine, is known (Non-Patent Documents 2 and 3).
Hosono as one of the present inventors, et al, previously found anew that such free oxygen can be replaced by various anions such as O2−, O− and OH− and filed patent applications with respect to inventions relating to the compound itself, a process for its production and applications for such a compound (Patent Documents 1 to 6).
Further, Hosono et al found that when a C12A7 powder as a mayenite type compound obtained by a solid phase reaction, or its sintered product, was subjected to heat treatment in a hydrogen atmosphere to prepare a C12A7 compound having H− taken in the cages, and then, the compound was irradiated with ultraviolet light to have electrons taken in the cages, it was possible to impart electroconductivity. And, they filed a patent application with respect to the invention relating to the compound itself, a process for its production and applications of such a compound.
However, by such a preparation method wherein H− is clathrated in the above sintered product, followed by irradiation with ultraviolet light, electrons are clathrated only at the surface portion of the sintered product irradiated with the ultraviolet light, and it was not possible to clathrate electrons into the interior of the powder or sintered product being a region not irradiated.
Further, Hosono et al developed a method for preparing a C12A7 single crystal and found it possible to impart electroconductivity to such a crystal by exposing the crystal to an alkaline vapor to have electrons clathrated in cages, and they filed a patent application with respect to the invention relating to the compound itself, a method for its production and applications of such a compound (Patent Document 6).
This preparation method utilizes a reaction to withdraw free oxygen from the C12A7 crystal in a solid state. However, in such a reaction, diffusion of oxygen in the interior of the solid became a rate-determining step, and it took a long time to have electrons clathrated in a sufficient amount.
On the other hand, the present inventors knew that in a molten state of C12A7 at a high temperature, the diffusion coefficient of the oxygen could be made high as compared with in a solid state, whereby the reaction to withdraw free oxygen proceeded quickly.
However, it was known that when a furnace circulating nitrogen was employed, from the melt having a C12A7 composition, a 3CaO.Al2O3 (hereinafter referred to as C3A) phase and a CaO.Al2O3 (hereinafter referred to as CA phase) could form as decomposition products and no C12A7 crystal would be formed. Thus, it was usually difficult to simultaneously carry out the reaction to withdraw oxygen and the reaction to form a C12A7 crystal (Non-Patent Document 4).
Hosono et al found that by reducing the surface area of the raw material by employing an isostatistic pressing product having a dense structure as compared with a powder, it was possible to make mild the reaction to withdraw oxygen in the temperature rising process being a reaction which took place at the surface, whereby it was possible to suppress formation of decomposition products, and they invented a method for preparing a C12A7 compound having oxygen in cages replaced by electrons, which comprises melting an isostatistic pressing product of C12A7 powder in a reducing atmosphere or in a covered carbon crucible and filed a patent application (Patent Document 6).
Patent Document 1: JP-A-2002-3218
Patent Document 2: JP-A-2003-40697
Patent Document 3: JP-A-2003-128415
Patent Document 4: JP-A-2002-316867
Patent Document 5: JP-A-2003-238149
Patent Document 6: JP-A-2004-26608
Patent Document 7: Japanese Patent Application No. 2003-183605
Non-Patent Document 1: H. B. Bartl and T. Scheller, Neuses Jarhrb. Minerai, Monatsh. (1970), 547
Non-Patent Document 2: P. P. Williams, Acta Crystallogr., Sec. B, 29, 1550 (1973)
Non-Patent Document 3: H. Pollmann, F. Kammerer, J. Goske, J. Neubauer, Friedrich-Alexander-Univ. Erlangen-Nurnberg, Germany, ICDD Grant-in-Aid, (1994)
Non-Patent Document 4: R. W. Nurse, J. H Welch, A. J. Majumdar, Transactions of the British Ceramic Society (1965), 64(9), 409-18